This invention relates to medical ultrasonic imaging methods using multiple-pulse sequences, and in particular to improvements to such imaging methods that improve contrast agent specificity.
As described below, the use of multiple transmit events to insonify a single direction is a preferred method for detecting harmonics of ultrasonic contrast agents. The incorporation of unequal amplitudes and/or phases between pulses within a sequence of two or more pulses has been used to suppress fundamental signal energy and to retain other order harmonics of the fundamental signal when the two or more receive pulses are combined properly. Adequate fundamental signal suppression provides improved contrast-to-tissue specificity, since tissue image clutter, or haze, is minimized and harmonics specific to the contrast agents are detected and displayed against a tissue background that is dark due to the lack of significant fundamental signals.
Acoustic reverberations can degrade the amount of fundamental signal suppression obtained with a multiple-pulse sequence for detecting contrast agents. Reverberant energy detected after a single transmit event that originated from an earlier transmit event can perturb the signal strengths of one or more of the received pulses such that the combination of multiple received pulses will not adequately suppress the fundamental signal energy.
More specifically, a number of multiple-pulse techniques have been introduced in the last few years that improve the detectability of ultrasonic contrast agents. The initial techniques simply used RF or I/Q filtering on a single receive pulse and only required a single transmit event. More advanced techniques combined this single-pulse filtering, or the lack of any filtering, with two or more pulses fired in the same direction. These techniques improve contrast agent specificity by using the multiple-pulse combinations to isolate a desired harmonic signal rather than relying solely on the RF filtering on each received pulse. This multiple-pulse approach offers improved signal bandwidth and improved spatial resolution, since harmonics of interest can overlap in the frequency domain with fundamental signals. The use of very broad single-pulse RF filters, or the lack of any RF filtering, retains the overlapping signal spectra, thereby preserving signal bandwidths. The multiple-pulse approaches also provide the potential to suppress other harmonics including the fundamental signal, while preserving a specific harmonic of interest. These techniques can also reject multiple harmonic orders including the fundamental, while preserving desired harmonics.
Prior multiple-pulse techniques that alter the transmit amplitude and/or phase between two or more transmit events for the purposes of detecting ultrasonic contrast agents include the following:
1. Pulse Inversion (U.S. Pat. Nos. 5,706,819 and 5,951,478) and Phase Inversion (U.S. Pat. No. 5,632,277)
These techniques do not vary the transmit amplitude between two firings, but transmit the opposite polarity on the second firing. With the summation of the two received pulses, fundamental signals as well as all odd order harmonics are suppressed while second order signals are retained.
2. Pulse Inversion Doppler (or Power Pulse Inversion) (U.S. Pat. No. 6,095,980)
This technique is an extension of Pulse Inversion that incorporates three or more transmit events where the amplitude does not change between firings but the polarity alternates between transmit firings. With properly selected receive weights, a combination of multiple receive pulses suppresses all odd order harmonics and retains second order signals. The use of additional transmit events and receive weights offers the potential for improved tissue flash suppression and improved contrast signal to electronic noise levels.
3. xe2x80x9cMeans for Increasing Sensitivity in Non-linear Ultrasound Imaging Systemsxe2x80x9d (U.S. Pat. No. 5,577,505)
This technique does not vary the transmit phase between two or more transmit events but varies the amplitude between transmit events. The weights, or scaling, on each received pulse are chosen such that the fundamental signals are suppressed and second harmonic signals are retained when all the received pulses are combined.
4. xe2x80x9cHigher Order Nonlinear Ultrasonic Imagingxe2x80x9d 1999 IEEE International Ultrasonics Symposium, October 17-20, Nevada, USA, Session: Non-linear Imaging, page 188 of Technical Program and Abstracts.
This technique varies transmit amplitudes and phases and chooses the appropriate receive weights to specifically isolate a single harmonic order.
5. xe2x80x9cMedical Diagnostic Ultrasound System Using Contrast Pulse Sequence Imagingxe2x80x9d (U.S. patent application Ser. No. 09/514,803) and xe2x80x9cMedical Ultrasonic Imaging Pulse Transmission Methodxe2x80x9d (U.S. patent application Ser. No. 09/650,942)
These techniques also vary the transmit amplitudes and phases between two or more transmit events to preferentially isolate single harmonic orders or two or more harmonic orders.
All these techniques use the idea of varying the transmit phase and/or amplitude between two or more pulses, but are susceptible to sub-optimal fundamental cancellation resulting from acoustic reverberations.
The methods described below transmit an additional pulse or pulses before the desired multiple-pulse sequence or incorporates modifications to a desired transmit pulse sequence across different scan lines such that the combination of received pulses exhibits improved fundamental signal suppression. The additional transmit pulse(s) are selected with the appropriate amplitude and phase to enhance fundamental signal suppression. For desired pulse sequences that are modified on a line-by-line basis, specific amplitudes and phases are chosen to properly suppress acoustic reverberations. This method can use any multiple-pulse sequence that varies the amplitude and/or phase between any two transmit pulses to achieve improved agent-to-tissue specificity and improved image quality when using ultrasonic contrast agents.